The field of the invention is magnetic resonance angiography ("MRA"), and particularly, dynamic studies of the human vasculature using contrast agents which enhance the NMR signals.
Diagnostic studies of the human vasculature have many medical applications. X-ray imaging methods such as digital subtraction angiography ("DSA") have found wide use in the visualization of the cardiovascular system, including the heart and associated blood vessels. Images showing the circulation of blood in the arteries and veins of the kidneys and the carotid arteries and veins of the neck and head have immense diagnostic utility. Unfortunately, however, these x-ray methods subject the patient to potentially harmful ionizing radiation and often require the use of an invasive catheter to inject a contrast agent into the vasculature to be imaged.
One of the advantages of these x-ray techniques is that image data can be acquired at a high rate (i.e. high temporal resolution) so that a sequence of images may be acquired during injection of the contrast agent. Such "dynamic studies" enable one to select the image in which the bolus of contrast agent is flowing through the vasculature of interest. Earlier images in the sequence may not have sufficient contrast in the suspect vasculature, and later images may become difficult to interpret as the contrast agent reaches veins and diffuses into surrounding tissues. Subtractive methods such as that disclosed in U.S. Pat. No. 4,204,225 entitled "Real-Time Digital X-ray Subtraction Imaging" may be used to significantly enhance the diagnostic usefulness of such images.
Magnetic resonance angiography (MRA) uses the nuclear magnetic resonance (NMR) phenomenon to produce images of the human vasculature.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B.sub.0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B.sub.1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, M.sub.z, may be rotated, or "tipped", into the x-y plane to produce a net transverse magnetic moment M.sub.t. A signal is emitted by the excited spins, and after the excitation signal B.sub.1 is terminated, this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (G.sub.x, G.sub.y and G.sub.z) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals, are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
To enhance the diagnostic capability of MRA a contrast agent such as gadolinium can be injected into the patient prior to the MRA scan. Several non-time resolved methods exist for coordinating acquisition of a single 3D image at peak arterial enhancement. In one method, a small test bolus of contrast agent is injected, and a time series of rapid 2D images are acquired. The 2D images are examined to determine the time when the contrast will arrive in the vessels of interest. The 2D temporal information is then used to delay the image acquisition relative to the injection of a full dose of contrast agent to achieve an optimal k-space acquisition. Similarly, flouro-triggering techniques use rapid 2D image acquisition to determine when the contrast is approaching the vessels of interest. When the monitor volume is seen to exhibit enhancement due to the contrast, the operator signals the scanner to switch from a 2D time series of images to a single high resolution 3D acquisition. Automatic triggering of the arrival of the contrast is also possible by acquiring an NMR projection and setting a threshold which switches the scanner to a 3D acquisition.
Single time frame 3D angiograms cannot display dynamic aspects of how blood vessels enhance, and therefore may lack some diagnostic information. If a particular angiogram contains blood vessels which fill later than others, in vessels distal to aneurysms for example, it is impossible to guarantee that both early and late filling vessels are optimally imaged.
An alternative method for acquiring angiograms is to acquire a series of "time resolved" volume images during the passage of the bolus of contrast agent. As described in U.S. Pat. No. 5,713,358, a series of images are acquired which depict the subject as the contrast agent enters the region being imaged. A reference image, or "mask," which depicts the subject before contrast agent arrives at the region of interest is subtracted from one of these images to remove the static tissues and further highlight the vasculature into which the contrast agent flows. The critical central k-space views are acquired every alternate time frame, thus assuring at least one set of central lines are acquired during peak contrast enhancement. The peripheral k-space lines are acquired less frequently and temporally interpolated in order to form a series of time resolved, 3D images. This method eliminates the need for timing the passage of the bolus of contrast, and this time-resolved method is thus less subject to operator error.
The current clinical implementation of this time-resolved method acquires either 15 or 20 high resolution 3D image frames. The large number of 3D volumes of data which are produced require significant computing power to reconstruct. Offline workstations are used for reconstruction, and typically reconstruct 10 time frames in no less than 1 hour for phased array data sets. There is no way, a priori, for the operator to know which time frame(s) will contain the peak arterial information. In addition, regions of k-space are combined without any knowledge of which regions were acquired during the peak of the contrast passage. Due to the long delay between acquisition of the data and display of the reconstructed images, physicians are not afforded the opportunity to review the results before the patient departs from the scanner.